Euthalenessa digitata (Mcintosh)

Pettibone, M. H., 1970, Revision of the genus Euthalenessa Darboux (Polychaeta: Sigalionidae), Smithsonian Contribution to Zoology 52 : 18-22

Euthalenessa digitata (Mcintosh)

Figures 12, 13

Thalenessa digitata Mcintosh , 1885, p. 140, pi. 22: fig. 2, pi. 23: figs. 5-7, pi. 13A: figs. 7-10, pi. 32A: fig. 9.-[?] Willey, 1905, p. 260, pi. 2: figs. 50-52.-[Not Hartman, 1966, p. 178.]

Euthalenessa digitata .-Daifooux, 1900, p. 116.-[Not Augener, 1927, p. 124.-Not Knox, 1960, p. 97.]

Thalenessa djiboutiensis Gravier , 1901, p. 231, pi. 7: figs. 114-117, text-figs. 240-248.

[?] Thalanessa [sic] digitata .-Potts, 1910, p. 351.

Euthalenessa djiboutiensis .-Fauvel, 1918, p. 331.-Wesenberg-Lund, 1949, p. 258.

Not Euthalanessa [sic] djiboutiensis .-Monro, 1937, p. 264.

Material examined.- Off Admiralty Islands, western Pacific , 29-45 meters, Challenger Expedition, March 1875 - holotype of Thalenessa digitata ( BMNH1885: 12: 1: 109a ) .

Djibouti, Red Sea , H. Coutière, collector, 1897-2 syntypes of Thalenessa djiboutiensis (MNHNP) .

Type-material.-The holotype of Thalessa digitata , now in 4 pieces, has a total length of about 126 mm, width, including setae, 6 mm, and about 160 segments (Figure 12). It is a female, filled widî eggs posteriorly.

The 2 syntypes of Thalenessa djiboutiensis are incomplete; the larger syntype, with the pharynx fully extended, has a length of 56 mm, width of 6 mm, including setae, and about 100 segments; the smaller syntype is about 40 mm long, width of 4 mm, including setae, and 64 segments (Figure 13).

Description.-The body is colorless. The elytra change in size and shape along the body. At first they are small and oval, then larger, subrectangular, subreniform to subcordiform, with deep anterior notches (Figures 12/-n; 13 a, h-j). The 1st pair of elytra lack papillae. The anterior elytra have fringes of papillae extending along most of their lateral borders, with 11-15 papillae; more posteriorly, the papillae are confined to the anterior halves of the lateral borders, with 12 to 8 papillae. The papillae have 1 to 3 filaments per papilla (up to 4, according to Mcintosh; up to 5, according to Gravier).

The pharynx has 11 pairs of papillae and 2 pairs of amber-colored jaws; subterminally a pair of diagonal muscular masses are found both dorsally and ventrally (syntype of T. djiboutiensis ).

The fused prostomium and tentacular segment are withdrawn within die anterior few setigers (Figures 12a, b; 13a). The 2 pairs of eyes are moderately large, located on the anterior slightly raised oval area of the prostomium, the anterior pair being larger than the posterior pair; the posterior half of the prostomium is covered dorsally by the anterior segments. The ceratophore of the median antenna extends from a wider median area between the anterior pair of eyes, narrowing more distally where it is fused to the dorsal bases of the fused tentacular parapodia. The ceratophores of the lateral antennae are short and also fused to die dorsal sides of die tentacular parapodia; they extend to about the same level as the median ceratophore; the 3 free antenna! styles are subequal in size, short, and subulate. The long palps extend posteriorly to setigers 11 to 16. The dorsal tentacular cirri are short and tapered; the ventral tentacular cirri are twice as long as the dorsal. The inner dorsal tentacular ridges are found on the distal diree-fourths of the tentacular lobes; setae are few or absent. The bulbous facial tubercle is visible ventrally medial to the inner palpal sheaths or anterior to die fused tentacular parapodia when die pharynx is extended; 3-A pairs of labial ctenidia are present on die lateral lips (Figures 126; 13a).

The parapodia of setigers 2-5 are greatly modified, with well-developed notopodial and neuropodial bracts (Figures I2 b-e; I3 b-c). The anterior and posterior notopodial bracts encircle die compact bundles of notosetae; tfiey are variously slashed, widi 4-5 anterior and 6-9 posterior stylodes. The neuropodial bracts consist of die following: (/) lower-anterior-ventral bracts, greatly enlarged and flaring; tiiey are largest on setigers 2 and 3, becoming shorter on setigers 4 and 5; tiiey have 2-4 papillae or stylodes on dieir upper distal borders and 0-1 stylodes on their lower distal borders; (2) upper-anterior bracts, shorter and wider; dieir lower distal parts form distinct rounded lobes or digitiform ligules, directed inward; (3) lower-posterior bracts, oval, with 2-4 distal papillae or stylodes; and (4) upper-posterior bracts formed of 8-10 digitiform stylodes. In die following transitional setigers, die lower-anterior and upper-anterior bracts become shorter and truncate, widi a notch between; die lower digitiform ligules of die upper-anterior bracts are prominent; die lower-posterior bracts become shorter, oval, lacking stylodes; die upper-posterior stylodes are fewer in number - 4-5 upper and 2 larger lower ones on setiger 6 (Figure 12/). By setiger 8 (holotype of T, digitata ) or setiger 15 (syntype of T. djiboutiensis ), die stylodes have disappeared and die posterior bract is continuous. The long, slender, compound neurosetae of die anterior setigers have multiarticulate blades widi 6-10 articles; some of die neurosetae are stouter, widi short blades; die stems are smooth or have few spinous rows (Figures 12d; 13c). The dorsal cirri on setiger 3 are short, subulate, borne on short cirrophores

(Figures 12c; 136). Small ctenidia encircle die elytriphores of setiger 2 and die dorsal cirri of setiger 3.

The branchiae begin on setigers 4-6. The parapodial ctenidia are large, cup-shaped, 3 per parapodium. The parapodia of die anterior segments have small clubshaped notopodia and larger neuropodia (Figures \2g-i; I3 d,e). The notopodial bracts have 1-3 posterior and 2-3 anterior stylodes. The notosetae form radier small bundles; they are slender, spinous, tapering to capillary tips. The neuropodial acicular lobes are enclosed in anterior and posterior bracts. The C-shaped posterior bracts are subtriangular and enclose die C-shaped row of neurosetae. The smaller lower-anterior bracts are truncate and enclose die lower diagonal row of neurosetae. The larger upper-anterior bracts are rounded, widi prominent digitiform lobes or ligules on dieir lower borders; die upper borders of die bracts curve around die upper diagonal row of neurosetae. The compound neurosetae are stout; the distal tips of die enlarged stems have few faint spinous rows; die blades are all radier short, widi bifid hooked tips (Figures 12i; 13e). The ventral cirri are slender, tapering, extending to die distal tips of die neuropodia.

The parapodia of die middle and posterior regions of die body differ in some respects (Figures 12;, k; 13/, g). The notopodia are similar, having a single posterior and 2-3 anterior stylodes. The neuropodia show marked changes, however. The posterior bracts are more elongate and diagonally truncate. The upper-anterior bracts are smaller and more or less fused widi die acicular lobes in die areas where die neurosetae are lacking; die lower ligules are conspicuous, appearing as oval lobes below die tips of die acicular lobes. The neurosetae are similar to die anterior neurosetae, die upper and lower ones having slightly longer blades (Figures \2k; 13g). The ventral cirri may be somewhat longer, extending beyond the distal tips of the neuropodia.

Distribution.-Western Pacific Ocean (Admiralty Islands, north of New Guinea), Red Sea, Persian Gulf, and Gulf of Oman, Maldives? Ceylon? In 10 to 45 meters.

Remarks.-The specimens reported by Willey (1905) from Ceylon and by Potts (1910) from the Maldives, as T. digitata , need to be reexamined. The record of E. djiboutiensis by Monro (1937) from the Maldives was based on a small specimen in poor condition( BMNH 1937: 9: 2: 68); the specimen was reexamined and the record is considered doubtful.